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Coil Springs Rate Explained – How to Find Quality Replacement Springs for Your Car

The Basics of Coil Springs and Their Impact on Suspension

Automotive suspension performs what is arguably one of the most important functions on a vehicle. Driving a vehicle without some sort of suspension would be impossible. That being said, the suspension is also one of the most tunable parts of your car. The order of suspension parts upgrades almost always starts with coil springs. However, that doesn’t mean that searching and finding the right parts is all there is to coil springs. There’s so much more to it. How do you choose the right coil springs from the vast catalog of available models? What are spring rates, linear and progressive springs? Let’s find out.

Intro to Coil Springs

The main purpose of automotive suspension is to keep the wheels on the ground at all times. That job is much harder than it sounds. Without suspension, every time you hit a bump or any form of road surface imperfection, your wheels would jump off the ground, thus losing grip. Depending on how fast you’re moving, that bounce could be catastrophic.

There are two main parts to any suspension system – a coil spring and a shock absorber. The latter is there to soak up the kinetic energy of a wheel that has just rolled over an obstacle, hence the name. The former is there to push the wheel back down towards the road surface, thus ensuring that your tires are constantly in contact with the road.

Intro to Coil Springs

How do Coil Springs Work?

The idea behind coil springs is a fairly simple one. You have a piece of metal wire that is wound in a coil spring. Coil springs are designed to be compressed, which ties into the subject of coil spring rates that we’ll discuss later on. The way coil springs compress can determine how stiff your ride will be and much more.

That being said, there is a lot that goes into building coil springs. Some top manufacturers, such as Lesjofors, have pushed material sciences to a whole new level. The same applies to the engineering aspect and a whole array of other processes that need to work together for a quality coil spring to be produced.

How do Coil Springs Work?

Aftermarket vs. OEM

Coil springs, much like most other mechanical vehicle components, wear out over time. The endless cycles of compression and expansion will bring any spring to its knees, no matter how well it is made or who made it.

Once that happens, you’re presented with two choices: either get OEM replacement springs or a set of aftermarket replacement springs that offer a different driving experience. If you choose the latter, there are a few things you should figure out first. One is spring rates, and the other is linear vs. progressive spring profile.

What is a Spring Rate?

Spring rate or spring compression rate is a measurement that tells you how much weight you need to apply to a spring to compress it. It is usually measured in kilograms per mm or pounds per inch, depending on who makes the springs. When you see a 10k rating on a spring, that means that you’ll need to apply 10 kilograms of weight to compress the spring 1 mm.

In practical terms, spring rate help define the stiffens of coil springs. The more weight you need to apply to spring to compress it, the stiffer it will be. An increase in stiffness translates to reduced body roll, squatting under acceleration, and nosediving during braking. All of these are associated with better handling. However, there is a limit.

Progressive Springs vs. Linear Springs

Setting up a vehicle for the track is a complicated job, but it is guided by a set of available and proven rules. These rules help when you’re trying to find the right path and the right parts. Setting up a vehicle for street use with an occasional track day is a much taller order, especially suspension wise.

Linear Springs

The reason for this is simple. Track cars are almost always sitting on linear springs. Linear springs offer the exact same spring rate throughout their range of travel.

This means that you can always anticipate how your suspension will behave on the track under different driving conditions. However, linear springs are often way too stiff for street use. No matter how big of a car enthusiast you are, riding on stiff linear springs in the city gets very old, very fast.

Progressive Springs

One of the solutions is to use progressive springs. Progressive springs have a variable spring rate depending on how far they’re compressed. The idea behind progressive springs is to offer relative comfort at lower speeds but increase the stiffness at higher speeds.

The problem with progressive springs is that you can’t anticipate their performance. Once pushed hard, you’ll definitely notice the change between the different spring rates as the coil springs go through their available range of motion. There are progressive springs that are better optimized in this regard, but most of them suffer from the same problem.

Can Springs Be Too Stiff?

They absolutely can, and it’s not just about comfort either! Most of those who are new to vehicle mods think that getting the stiffest possible springs is the way to go. However, that approach is usually counterproductive. As mentioned before, the primary purpose of your springs is to keep the wheel in contact with the road and prevent it from jumping around.

A spring with a lower compression rate will compress more when a wheel hits something on the road. Being able to compress means that your wheel can negotiate the obstacle and adapt to the anomaly while maintaining some amount of grip. As soon as the wheel is over the bump, it will jump, and the springs will expand to compensate.

When you install springs that are too stiff, you’re essentially preventing the suspension from soaking up the bump. Instead, when the wheel hits a bump, the spring won’t compress, and you’ll have a wheel that is catching air. As we all know, an airborne wheel offers exactly no grip at all.

How to Figure Out The Right Spring Rate for Your Car?

If you thought that finding the right spring rate for your vehicle is going to be easy, you better turn back now. Whenever you’re dealing with suspension tuning, ‘easy’ flies out the window pretty much immediately.

The first thing you should understand is that the spring rate is just one of the variables in a much larger equation. Many different factors are at play when it comes to tuning the suspension and getting the most amount of grip.

Things such as sprung weight of the car, suspension geometry, wheel size and rate, flexibility of your chassis, the damping rate of your shocks all need to work together in unison.

There is a silver lining to this story, though. Specific vehicle models such as the BMW M3 or similar iconic cars are no longer a mystery as far as suspension tuning goes. There is enough material out there to help you dial in your suspension without having to spend lots of time and money figuring things out on the go.

Decisions

Figuring out the right set of coil springs is tough but not impossible. The point of this guide wasn’t to put you off from getting shorter, stiffer springs. Instead, our intended point was to show you just how complicated this job is if you want to do it the right way.

Figure out what you want to do with your vehicle, and search for a set of springs that match your intended application. The jump between OEM and aftermarket is usually massive, so expect your view of your car to change along with any replacement suspension parts you install.

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7 thoughts on “Coil Springs Rate Explained – How to Find Quality Replacement Springs for Your Car

  1. I agree with your findings of initial spring rates and ride heights for aftermarket springs. When new, they seem to be almost identical to their OEM counterparts.

    I’ve found is that in the harsh New England weather, aftermarket springs rust more quickly than OEM, and break at the rust spot. Can you comment on the thickness or quality of the paint on these aftermarket alternatives versus SAAB OEM?

    1. We are currently on the hunt for more OEM style aftermarket replacement springs for the 9-5, but right now we don’t have anything to compare to but Genuine SAAB to Genuine SAAB. If we do compare these together, that would be involved in a separate test.

  2. In Wisconsin, our roads are a disastrous mess of potholes and repairs. My ’04 9-3 Arc handles great, but it rides like a brick.
    Is there a spring and shock combination that can give my car a less jarring ride? Something less sporty, more like a luxury car ride?

    1. New Base/Linear springs will give you the most comfortable ride and also highest ground clearance. This test was to test the rates between OEM and Aftermarket, which we found to be very similar, and worth it to buy quality aftermarket options over OEM, which are costly and becoming harder to source. Also take into consideration that your tire size also greatly effects ride quality, you may want to try a smaller rim, larger tire option, like a 16″ wheel. It should be noted that springs sag with age, and even if your car was originally a non-aero, non-sport model, old sagging springs will cause changes in the quality of the suspension ride.

  3. my suspension and brake code is 42-12-4-3-62-60-AB-BA-CB-DA…what kind of suspension do i have can someone help

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